magnifier: Disable desaturation effect when not in use
The desaturation effect was always enabled, regardless of the specified factor. This was causing the magnifier to always hit offscreen rendering, even when not necessary. Additionally offscreen rendering is currently also causing glitches in MetaWindowGroup which this helps to avoid in some cases.
Related: mutter#1678 (closed)
This is 40 and 41 material
Edited by Sebastian Keller